Electrical: Split charge current

[jospanner]

Hi,
I'm redoing the electrics on an old motorhome.
I know that the alternator isn't a good way to charge the leisure battery, but getting some charge off it while driving seems like a good idea. So, I was thinking about a voltage sensing relay to split the charge system, with 16mm² cables connecting to the leisure battery.

My question, though: If my alternator supplies around 75 amps, is that going to damage the leisure battery? Aren't these batteries normally charged in the single digits?

Maybe I'm being silly here, but I see talk of how the relay must be able to handle the high current off the alternator, but also talk about how the battery should be charged at this low amperage, so I don't understand what's happening here.

 

[chaser]

Hi,
I'm redoing the electrics on an old motorhome.
I know that the alternator isn't a good way to charge the leisure battery, but getting some charge off it while driving seems like a good idea. So, I was thinking about a voltage sensing relay to split the charge system, with 16mm² cables connecting to the leisure battery.

My question, though: If my alternator supplies around 75 amps, is that going to damage the leisure battery? Aren't these batteries normally charged in the single digits?

Maybe I'm being silly here, but I see talk of how the relay must be able to handle the high current off the alternator, but also talk about how the battery should be charged at this low amperage, so I don't understand what's happening here.

Don't understand parts of your post, but a leasure battery is no different to your starter battery when it comes to charging.

 

[chatteris]

the alternator regulates the current going to the battery so will not overcharge unless it is faulty

 

[andy63]

So, I was thinking about a voltage sensing relay to split the charge system, with 16mm² cables connecting to the leisure battery.

I ran a system like that to provide a charge to the leisure battery for a good number of years and found it satisfactory, while accepting it may not be as efficient as a dedicated battery to battery charger..
Just make sure you get a good rated relay..I used a 120A one.. And it was dual sensing... So whatever bank was receiving a charge the other battery got something as well once the voltage threshold was reached...
Andy..

 

[Lenny HB]

I wouldn't use a voltage sensing relay but a standard relay switched by the D+ terminal on the alternator.

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[CamperJack]

From what I understand of these things a relay doesn’t work in newer engines. You say your MH is older, but I’d quantify that and make sure a relay will work for you. I know a relay wouldn’t work in my van so I ended up having to pay a lot more for a Batery to Batery charger.

 

[Boris7]

This may help

Split Charging Guide - caravans, campervans, motorhomes, boats, vehicles

Our guide to split charging for caravans, campervans, motorhomes, boats & commercial vehicles. A look at how it works & the various technologies available.

www.12voltplanet.co.uk

 

[jospanner]

I wouldn't use a voltage sensing relay but a standard relay switched by the D+ terminal on the alternator.

The old system did have a relay that appeared to be switched by the ignition, it didn't seem to function though.

From what I understand of these things a relay doesn’t work in newer engines. You say your MH is older, but I’d quantify that and make sure a relay will work for you. I know a relay wouldn’t work in my van so I ended up having to pay a lot more for a Batery to Batery charger.

It's a 1991 Autotrail, based on a Mercedes 208D.
So it's got a very basic electrical system - these vans were designed in the 70s!

the alternator regulates the current going to the battery so will not overcharge unless it is faulty

Thank you, that's good to know. I suppose it makes sense, otherwise it would keep killing the vehicle battery.

 

[Boris7]

The old system did have a relay that appeared to be switched by the ignition, it didn't seem to function though.

l suspect thats the issue you need to sort, a simple multimeter will confirm the relays operation (or not) if it’s dead then replace with a like for like or another quality relay.

Car builder solutions are a good place to start, if the wiring diagrams in the link I sent don’t fully explain the relays function then Pop back here

 

[Boris7]

If you. Don’t have a multimeter then its worth buying one, but don’t be fooled into spending loads, but make sure it has a continuity function with sound alert.

if I’m telling you how to suck eggs then feel free to say so

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[chatteris]

Might just have a blown fuse I somehow managed to blow the charge fuse on my bessacar while doing some additional wiring

 

[Treeeev]

I had standard 30A split charge relays in two VW’s (T2 Devon Eurovette and T3 Autohomes Kamper) over a total period of approx 24 years. These worked ok and the only problem I had with them was the fuse blew a couple of times when the leisure battery had been run down. In my self build VW T4, I fitted a 100A voltage sensing relay. This worked without any problems for over 10 years until I sold it.

One advantage of the voltage sensing relays are that most are bi-directional, ie the leisure battery is charged when the engine is running, and conversely, the engine battery is charged when on EHU and the charger is on. The only downside I found ( very minor) was that sometimes there was a chattering of the relay. This was easily stopped by blipping the throttle if the engine was running, or if on EHU just turn something 12v on for a short while until the voltage was raised a little.

If it wasn’t just a blown fuse, my suggestion in this instance as it is an older van, would be for a VSR.

Cheers

Trevor

 

[autorouter]

My question, though: If my alternator supplies around 75 amps, is that going to damage the leisure battery? Aren't these batteries normally charged in the single digits?

It depends on the battery type. A gel battery normally has a maximum recommended charge rate of C/5, where C is the Ah capacity. So for example a 100Ah gel should be limited to a continuous charge rate of 100/5 = 20 amps. For a 75 amp alternator, also charging the starter battery, it's unlikely to exceed that significantly. The alternator has a built-in regulator. Other battery types are less sensitive to higher charge rates.

Voltage sensitive relays work fine in simple systems, but can have unintended problems if you add solar or run from ehu, ie when the starter battery voltage is raised by other things than the alternator.

 

[jospanner]

If you. Don’t have a multimeter then its worth buying one, but don’t be fooled into spending loads, but make sure it has a continuity function with sound alert.

if I’m telling you how to suck eggs then feel free to say so

I have a meter. I've tested the relay myself, and it seems to be OK. The wiring under the dash was a mess and I've cleaned a lot of it up.

One issue I had was the starter battery seemed to be dying. I'm going to have another look at the wiring off the alternator. I assumed it was charging correctly and I just had a duff starter battery, because the engine was running. It only just occurred to me that mechanical injection Diesel engines can run without a working electrical system.

It depends on the battery type. ....

Voltage sensitive relays work fine in simple systems, but can have unintended problems if you add solar or run from ehu, ie when the starter battery voltage is raised by other things than the alternator.

I'm looking at running a lead acid based system, using maybe Varta batteries. I understand that they charge in the same manner as a lead acid starter battery? If the solar is hooked up to the leisure battery side of the circuit, wouldn't that charge both of them, rather than accidentally isolate the solar?

 

[autorouter]

There's several other considerations. You want to be sure that running the leisure battery flat won't discharge the starter battery too. The VSR should manage that OK. Also you want to be sure the leisure battery does not try to supply the very large current that the starter battery sends to the starter motor. A VSR will cut off, but will it do that quickly enough? If there's a diode to prevent back-flow, do you need another device (BatteryMaster?) to charge the starter battery when the leisure battery is charging?

Some people use VSRs with no problem, others seem to have problems. I'm just saying be careful.

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